The polymerization of lactams is described by Jan Sebenda in J. Macromol. Sci. Chem. A 6 (6), pp 1145-1199 (1972).
The anionic polymerization catalyst may be, for example, a mixture of sodium hydride and acetanilide, or a mixture of sodium and N-acetylcaprolactam.
The anionic polymerization of lactams is rapid, about 3 to 15 minutes; however, the inherent viscosity of the polyamide obtained is not stable and changes over times.
DE 2,241,131 describes the addition of diethyl melonate or of ethyl acetylacetate in the nylon-12 obtained by anionic polymerization of lactam 12 in order to avoid the large variations in the hot flow index (MFI) during its conversion into fibres, films or moulded articles.
DE 2,241,132 describes the addition of tert-butyl alcohol for the same problem. These results are not sufficient and, in addition, these additives migrate over time so that the stabilization is merely temporary.
K. Ueda, M. Nakai, M. Hosoda and K. Tai have described in Polymer Journal Vol. 28,, No. 12, pages 1084-1089 (1996) the instability of nylon-6 [or PA-6] obtained by anionic polymerization of caprolactam. They describe dissolving the PA-6 in DMSO at 150.degree. C. under nitrogen in order to remove the residues of the anionic catalyst by treatment with an acid having a pK.sub.a of 3 to 7 in water. This method can only be used in the laboratory.